1. Field of the Invention
The present invention relates to a liquid crystal display device of multi-domain mode and a fabrication method thereof, and a hard mask used for the same, and particularly, a liquid crystal display device having both a wide viewing angle and rapid response property and a fabrication method thereof, and a hard mask used for the same.
2. Description of the Related Art
A liquid crystal display device using a nematic liquid crystal was first used in a numerical segment display device such as a clock, a calculator, and the like. Now, it has become more widely used as a display device for a monitor of a notebook-sized personal computer (PC) or a desktop personal computer, having the advantages of a small occupying space and low power consumption. Particularly, in the market of a display device for a monitor of a desktop PC, a conventional cathode-ray tube (CRT) monitor is being replaced by an LCD monitor. Such a trend is also seen in the market of televisions, in which the CRT has been dominant. Manufactures are increasing research and development to replace a CRT-TV with an LCD-TV.
For an LCD-TV to be more widely used, a rapid response property which may support a motion picture, and a wide viewing angle which does not depend on an angle at which a viewer watches a screen, are the major issues. These are essential problems which are related to an electro-optic property of a liquid crystal, and various proposals have been submitted. However, a liquid crystal display which can provide both a rapid response property and a wide viewing angle is not embodied at the moment.
For example, regarding the rapid response property, among a liquid crystal display mode using a nematic liquid crystal, an optically compensated birefringence (OCB) mode disclosed in Japanese Laid-Open Publication No. 11-7018 is the best. In the OCB mode, when a predetermined voltage is applied to a liquid crystal layer, liquid crystal molecules near an alignment film are inclined symmetrically at both substrates to form a bend alignment. Since the bend alignment is formed in this way, a change of liquid crystal molecules, which corresponds to a change in a driving voltage applied to the liquid crystal layer, becomes fast, and a rapid response property, for example, a few msec, can be achieved. However, this OCB mode still has problems in terms of front contrast, viewing angle, uniformity of display, or the like.
On the other hand, in a liquid crystal display device disclosed in Japanese Laid-Open Publication No. 10-301113, liquid crystal molecules are aligned vertical to a surface of a substrate without a driving voltage being applied to a liquid crystal layer. Thus, a leakage of light is not likely to occur in a black display state, and the display device has a high contrast property. Furthermore, in a liquid crystal display device disclosed in Japanese Laid-Open Publication No. 10-301113, two domains each have liquid crystal molecules arranged at an angle different from each other by 180°. In a border region between these two domains, a small domain having liquid crystal molecules arranged in a direction perpendicular to both of the directions of the liquid crystal molecules of the above two domains is formed. In this way, a property of a viewing angle and a response property are improved.
Japanese Laid-Open Publication No. 11-352486 discloses a liquid crystal display device of a vertical alignment (VA) mode. In this liquid crystal display device, regions having alignment regulation forces in different directions are provided on vertical alignment films on a pair of substrates, which oppose each other with a liquid crystal layer sandwiched in between, by a mask rubbing process, or the like, to form four domains having different alignment directions of liquid crystal molecules positioned near the middle of a thickness direction of the liquid crystal layer when a voltage is applied. Japanese Laid-Open Publication No. 11-352486 describes that, mainly, any of a mask rubbing method by a photolithograpy method on a vertical alignment film, an ion beam method, and a light illumination method may be used as a pixel separation method. Japanese Laid-Open Publication No. 11-352486 describes that by forming domains having different alignment directions of liquid crystal molecules, a liquid crystal display device having an improved viewing angle property, and which can display at a high definition, can be achieved.
In Japanese Laid-Open Publication No. 2002-277877, a picture element has quartered domains. Therein, four domains having different alignment directions of liquid crystal molecules positioned near the middle of a thickness direction of a liquid crystal layer to be twist-aligned positioned sequentially in one direction. Substrates are bonded to each other such that a border of alignment regions on one substrate overlaps an alignment region of the other substrate vertically. Thus, a quartered domains RTN mode having different alignment states of the liquid crystal molecules is implemented. Japanese Laid-Open Publication No. 2002-277877 describes that, by combining vertical alignment and alignment split as such, a viewing angle can be increased, a display definition can be improved, and contrast can be improved.
Japanese Laid-Open Publication No. 7-2394747 describes a fabrication method of a liquid crystal display device of a multi-domain TN mode using a horizontal alignment film. In the method, at least one of a pair of substrates is subjected to a rubbing process at least once, the rubbing process is performed using a rubbing cloth through a plate having a mask portion having a stripe shape (non-opened portion) and an opening. For example, an opening provided by setting a stainless steel mask with tensions applied from both sides, and thus, the stainless steel mask being slightly lifted above the substrate can be used.
However, the present inventors have found that the conventional liquid crystal display device disclosed in Japanese Laid-Open Publication No. 10-301113 has the problem that a disclination line is generated between domains by applying/cutting of a voltage. As a result, there is a problem that, when a display surface is viewed at an angle, a display is seen to have a rough surface from any direction. Further, if a disclination line is generated badly, there is a problem that a display definition depends on a viewing angle.
In the conventional liquid crystal display device of Japanese Laid-Open Publication Nos. 10-301113, 11-352486, and 2002-277877, a pixel is split (domain is formed) to achieve a wide viewing angle. An alignment process for that purpose is mainly performed by a method of forming a pattern of a resist film by a photolithography method on a vertical alignment film and performing a mask rubbing process.
However, a rubbing method using a resist film with a pattern formed by a photolithography process requires a chemical process for developing or peeling a resist, and the like (a process to remove a portion to be rubbed by etching or the like). This causes damage on an alignment film surface, and an alignment regulation force cannot be controlled well. The present inventors conducted a detailed study, and have found that, in a portion of an alignment film which is protected by a photoresist and a portion which is not protected, the states of interfaces between an alignment film and a liquid crystal layer are different, and tilt angles and anchoring properties are different. This results in a difference in response properties and/or a difference in voltage/transmittance properties between split domains. Thus, the response property has not been improved as expected. Particularly, at a low temperature, for example, around 0° C., the response becomes significantly slow. Thus, it is shown that the above liquid crystal display device is not suitable for use as a liquid crystal display television.
As a method for performing an alignment process, a method of performing light illumination using an photo alignment film material having functional groups which react to ultraviolet rays having a certain wavelength or a bias direction, for example, can also be considered. However, an photo alignment film has problems that its reliability, retention property, or the like, is low.
Japanese Laid-Open Publication No. 11-352486 describes a mask rubbing process using a metal hard mask. Japanese Laid-Open Publication No. 2002-277877 describes performing a mask rubbing process with the necessary regions covered with a mask. However, a method using a resist film with a pattern formed by a photolithography process as a mask is mainly used. Currently, improving a response property, reliability, and the like, by performing a rubbing process using only a hard mask, as in the present invention, is not considered at all.
In Japanese Laid-Open Publication No. 7-2394747, in order to suppress damages of a stainless steel mask caused by the rubbing cloth, two ends of the stainless steel mask are pulled by a tension application mechanism (for example, a spring) and tension is applied on a substrate. As a result, a rubbing process is performed on the alignment film surface through an opening of the stainless steel mask with the stainless steel mask slightly lifted from an alignment film surface, and, particularly, a rubbing process region becomes wider than the width of the actual opening on both sides of the width direction relative to a rubbing direction of the opening of the stainless steel mask. Thus, there is a problem that a dimension precision cannot be achieved in the rubbing process region.